1. Field of the Invention
This invention relates to cameras having a built-in lens capable of focal length changeability and more particularly to the motorized change of the focal length of a photographic lens in such camera.
2. Description of the Prior Art
The prior-known mechanisms for changing the focal length of the photographic lens of a camera between two different values, as, for example, disclosed in U.S. Pat. No. 3,388,650, are adapted to be used with optical systems of such types in which: the combination of the lens components is altered (for example, the rear converter type); a lens of positive power and a lens of negative power are moved according to a prescribed rule (for example, the zoom lens type); and short and long focal length lenses are interchangable.
Since the operation of changing the focal length of any of these types of optical systems requires a distance which is far longer than that of the focusing and a very complicated manner, automation of the focal length changing operation through the use of a simple structure such as the one used in the focusing operation which is automated by use of a spring, has been difficult to achieve.
Therefore, most of the focal length changing mechanisms have been operated either with the help of a force from the outside, such that an actuator provided on the photographic lens barrel or on the camera housing is moved manually (for example, Japanese Patent Publication No. Sho 56-50504), or with the use of an electric motor solely adapted for that purpose (for example, Japanese Laid-Open Patent Application No. Sho 57-146232).
Also, in application of this motorization of the photographic lens to cameras of the type in which the winding and rewinding of film is carried out by a separate electrical motor, problems arise in relation to the cost and the availability of space within the camera housing, leading to an increase in the production cost as well as the bulk and size of the camera. To create a motor driven focal length changing device for this type of camera, many difficult problems have been left unsolved.